1. Field of the Invention
The present invention relates to non-reciprocal circuit elements, such as isolators and circulators used in the microwave band, and to communication devices comprising the same.
2. Description of the Related Art
A known lumped-constant isolator, or non-reciprocal circuit element, used for mobile communication devices such as cellular phones generally comprises a center electrode assembly, a magnet, a yoke, a resistor, and matching capacitors. The matching capacitors generally have Ag electrodes disposed on both surfaces of a dielectric substrate, as disclosed in Japanese Unexamined Patent Application Publication No. 11-186814.
The Ag electrodes are formed by screen-printing Ag paste so as to have a large thickness of 10 to 30 xcexcm, and then by firing the printed paste at 850xc2x0 C.
However, a burr or a crack may occur in the electrodes of the known capacitor when the capacitor is cut out. In other words, dicing the capacitor inevitably causes a burr in the thick electrodes, which can lead to several problems.
For example, the burr may cause the capacitance to be too low. Also, the burr effectively changes the size of the capacitor, which may prevent it from fitting accurately into a non-reciprocal device such as an isolator or circulator. Further, the burr may touch another capacitor or another conductor, causing a short circuit.
Moreover, a thick electrode, especially one made of metal, is soft and malleable. Thus, the thick electrodes readily cause clogging in the dicing blade and thus make the blade blunt. Furthermore, frit contained in the Ag paste remains after firing and adversely affects solder wettability.
In contrast, a dielectric ceramic substrate comprised in a capacitor is hard but brittle and fragile. Thus, it is difficult to select a dicing blade which will prevent cracking the substrate while avoiding the above problems of the electrodes, since the substrate and the electrodes respond so differently to the cutting operation.
Accordingly, the present invention provides a non-reciprocal circuit element comprising matching capacitors which are less susceptible to the problems described above, and provides a communication device comprising the non-reciprocal circuit element.
To this end, according to one aspect of the present invention, there is provided a non-reciprocal circuit element comprising matching capacitors. Each matching capacitor is disposed between an I/O port and ground, and comprises a dielectric substrate. First buffer layers, second buffer layers, and main lead layers are formed on both surfaces of the dielectric substrate by dry thin-film deposition, in that order. The first buffer layers are formed of a material which adheres to the dielectric substrate. The second buffer layers are formed of a material capable of preventing solder from diffusing into the dielectric substrate. The main lead layers are formed of a solderable conductive material.
The first buffer layers, the second buffer layers, and the main lead layers may be formed of various materials as long as the materials are capable of being used in dry thin-film deposition, such as sputtering, vapor deposition, thermal spraying, or ion plating. The first buffer layers may be formed of Ti, W, Ta, and Cr, and preferably a Nixe2x80x94Cr alloy. The second buffer layers may be formed of Nixe2x80x94Ag, Nixe2x80x94Au, Nixe2x80x94Ti, and alloys of the materials of the first buffer layers and the main lead layers, and preferably a Nixe2x80x94Cu alloy. The main lead layers may be formed of Cu and Au, and preferably Ag.
In this non-reciprocal circuit element, the matching capacitor has thin electrodes, that is, the first buffer layers, the second buffer layers and the main lead layers, and therefore it can be readily diced while preventing burrs and cracks. Also, since the electrodes of the matching capacitor are formed of suitable materials, the capacitor has excellent electrical characteristics, consequently allowing a non-reciprocal circuit element to have improved electrical characteristics. Since the electrodes are formed by dry thin-film deposition, the electrodes do not contain frit and thus solder wettability is improved. In addition, since the electrodes are formed by dry thin-film deposition, small amounts of materials can be used for the electrodes and the firing process of the electrodes can be eliminated.
According to another aspect of the invention, a capacitor having the above-described features is provided.
In a further aspect of the present invention, a communication device comprising the non-reciprocal circuit element is provided and the communication device shows preferred electrical characteristics.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.